The use of dielectric ceramic compositions for their desirable electrical properties is well known in the art. When such compositions are employed, certain electrical properties become particularly important. Material Scientists are then forced to balance the desire for certain electrical performance values against the need for materials that are easily manufacturable at reasonable temperatures and which may be easily scaled up for large volume manufacturing applications.
One important electrical property is the electrical Q (also referred to as "Q" or "Q-value") of a fired ceramic component. Electrical Q has a significant effect on the speed and quality of a soundwave through a ceramic component. As one typical application for these ceramic compositions is to provide frequency selectivity through filtering, a very high Q property is desired. When these ceramic dielectric compositions are used to form ceramic block filters, for example, they are pressed and fired with resonator through-holes formed therein. In such instances, the need for a high Q material is paramount.
Electrical Q may be dependent on a host of material properties, and may be drastically reduced by impurities, cracks, or poor grain boundaries in the ceramic composition. When a soundwave encounters such impediments as are described above, the soundwave will naturally slow in speed and require additional energy to effectively pass through the material composition. Conversely, a high-purity, well-processed ceramic composition may have a very high Q-values due to lack of impurities, cracks, and grain boundaries, resulting in efficient transport of an electronic signal through the material.
To a designer of electronic signal processing equipment such as cellular telephones, the importance of high electrical Q cannot be understated. High Q ceramic compositions result in components which use less power, thus increasing battery life and reducing the weight and volume of electronic devices. Additionally, a high Q ceramic composition may result in better overall performance by reducing background noise and increasing clarity in a cellular telephone, for example.
Electrical Q is intimately related to the insertion loss property of a ceramic filter, another important electrical property of interest to designers. For ceramic filter applications, high Q compositions provide filter frequency response curves which may have a narrower bandwidth and a correspondingly low insertion loss value. The desired low insertion loss specification may be met through other design techniques, however, if low insertion loss may be achieved at the material composition stage, designers are not forced to add extensive, labor and process intensive features in order to achieve the desired properties and performance.
Another important electrical property of any dielectric ceramic composition used for signal processing applications is the Temperature Coefficient of Frequency (T.sub.f). In simple terms, this property is a measure of how much the frequency of a signal will shift as a function of temperature. As telecommunication devices typically require that specific signals remain in very narrow, predetermined frequency ranges, a most preferable (T.sub.f) value would be zero. However, for most applications, a (T.sub.f) range of +/-10 ppm/.degree. C. is acceptable.
Still another important electrical property of a dielectric ceramic composition is its dielectric constant (K) (sometimes referred to as ".epsilon..sub.r "). The dielectric constant is a unitless measure of a material. The effect of different materials is compared to that of air, that is, if a capacitor has a given capacitance when air is used as a dielectric, other materials used instead of air will multiply the capacitance by a certain amount called the dielectric constant (K). For ceramic dielectric compositions used in the electronics industry, and in the ceramic filter industry in particular, dielectric constant (K) values in the range of 80-100 are desired.
A perusal of the patent literature finds a few patents which address these issues with various dielectric ceramic compositions. One patent in this area of technology is U.S. Pat. No. 4,769,354 issued on Sep. 6, 1988 to inventors Beauger et al. for a "Type I Dielectric Composition Based on Neodymium Titanate". While this composition may be used in the manufacture of multilayer ceramic capacitors and the like, it also contains lead titanate (PbTiO3) which cannot be used in certain domestic manufacturing operations for environmental reasons. Moreover, this patent addresses the issue of providing a composition which frits at high temperatures in the range of 1300.degree. C. These factors render this composition unacceptable for applicant's intended application.
U.S. Pat. No. 5,750,452 issued on May 12, 1998 to inventors Park et al. for a "Dielectric Ceramic Composition for Microwave" also addresses the need for a high Q, high dielectric constant, low T.sub.f material, but offers a composition which contains BaO, Sm.sub.2 O.sub.3, TiO.sub.2 and PbO. The lead content (PbO) of this material renders it unacceptable for applicant's intended application and manufacturing processes.
U.S. Pat. No. 5,077,247 issued on Dec. 31, 1991 to inventors Sato et al. describes a "Dielectric Ceramic for Microwave Applications". This composition may be readily distinguished from the applicant's invention because the Sato patent teaches the use of a BaO--TiO.sub.2 composition which is completely different from applicant's BaO--Nd.sub.2 O.sub.3 system.
U.S. Pat. No. 5,256,639 issued on Oct. 26, 1993 to inventors Fujimaru et al. describes a "Dielectric Ceramic Composition". This composition may be readily distinguished from the applicant's invention because the Fujimaru patent teaches the use of different amounts of the individual compositional elements; as well as a much lesser amount of samarium oxide (Sm.sub.2 O.sub.3).
U.S. Pat. No. 5,493,262 issued on Feb. 20, 1996 to inventors Abe et al. describes a "Dielectric Ceramic Composition Containing ZNO--B.sub.2 O.sub.3 --SiO.sub.2 Glass, Method of Preparing the Same, and Resonator and Filter using the Dielectric Ceramic Composition". This composition may be readily distinguished from the applicant's invention because the Abe patent also teaches the use of different compositional elements in a method type patent which encompasses some rare earth metal oxide materials.
A high purity dielectric ceramic composition which provided improved electrical properties in the form of ultra-high electrical Q, a low (T.sub.f) property and a high dielectric constant (K) while simultaneously being amenable to large scale manufacturing processes and operations and which contained a mixture of materials which effectively provided these desirable properties in a custom composition of various oxide materials advantageously including samarium oxide (Sm.sub.2 O.sub.3) would be considered an improvement in the art.